Glasses are known that are absorbing in the infrared spectral range (wavelength range about 800 nm to 2 μm) and are light-transmitting in the visible spectral range (wavelength range 400 nm to 800 nm). These so-called heat protection glasses or short-pass filter glasses are normally iron-doped glasses. Depending on the manufacturing conditions (raw materials, glass matrix, melting temperature and atmosphere), iron is incorporated in glasses as metallic iron (colloidal), as Fe2+, or as Fe3+. Fe3+ exhibits absorption bands in the ultraviolet range at about 380 nm and 410 nm, the extensions of which in the visible range lead to a brownish yellow coloration. By contrast, the Fe2+ ion has overlapping absorption bands in the infrared range (at about 1000 nm and at 2000 nm), the extensions of which into the visible range give the glass a blue color. If both oxidation stages are present, the glass appears to be greenish. The alkali ions that are present in ordinary soda-lime-silica glasses shift the balance between Fe2+ and Fe3+ in favor of Fe3+, which limits the use of such glasses as heat protection glasses because of the relatively poor transmission in the visible spectral range.
In a known method for producing iron-doped glass, a porous silicate glass is impregnated with an iron salt solution. The starting materials are borosilicate glasses that are leached for producing the porous SiO2 skeleton, but residues of boron remaining in the glass strongly reduce the heat resistance. Moreover, depending on the process management, iron is present in the form of Fe3+ and Fe2+ ions during production of these glasses; the relatively high amount of Fe3+ reduces light transmission in the visible spectral range and the use as a heat protection glass is therefore unsatisfactory.
On account of these problems, heat protection glasses have been suggested that are based on quartz glass as the starting substance and are doped with iron (hereinafter also called “iron-doped silica glass”).
Such glasses are known from U.S. Pat. No. 4,500,642 A and U.S. Pat. No. 4,419,118 A, which describe the quartz glass being doped with iron and aluminum. The corresponding oxides are added in powder form during fusion of a natural quartz grain in an oxyhydrogen flame or an electric arc. The block-shaped blanks obtained thereby are subjected to a post-treatment in a hydrogen-containing atmosphere after fusion, so that the oxidation state of the iron is changed in favor of Fe2+ ions. As a result of the post-treatment, the color of the Fe-doped silica glass changes from more or less brown at the beginning to clear transparent-turquoise blue. This change is also confirmed by the transmission measurement in the visible or infrared spectral range. The fused Fe-doped silica glass without post-treatment in hydrogen-containing atmosphere only shows a transmission of not more than 20% in the visible spectral range, this transmission rising to about 40% in the infrared range, whereas the sample after the treatment with hydrogen shows a transmission of more than 80% in the visible spectral range—with an approximately equal absorbing behavior (up to 40%) in the infrared range. According to this prior art, co-doping with aluminum is further considered to be necessary because in the absence of aluminum the Fe-doped silica glass is not sufficiently heat-resistant and a dark coloration of the glass will occur upon permanent use. The latter is due to a stabilizing effect in favor of the oxidation form Fe2+ by the multivalent aluminum.
The method according to the prior art requires a time-consuming post-treatment on the already vitrified blank. The diffusion of the hydrogen, which must penetrate through the blank, is decisive for the method. Especially in the case of large-volume blanks, the post-treatment does not yield a sufficiently homogeneous reduction to Fe2+. Another disadvantage of the method according to the prior art is the addition of a second dopant because a homogeneous distribution of the second dopant and the reproducible adjustment of the oxidation state of the iron poses problems due to the second dopant.